Two Sites on my Webspace?

[#1]

Can I use my PlusNet webspace to host two separate websites?  I am a PlusNet user with 250 Mb website allowance.  I have one site that uses about half of my allowance.
Included in that site are pages that I would eventually like to split off to host under a domain.  I can see when the time comes, it will not be straightforward to separate out those pages.
If they were part of their own site, it would be a lot easier to upload them elsewhere.

[#2]

Yes you can, either free or by paying the hosting charge depending on the hosting allowance for your product. Of course the storage and data transfer allowance is shared by all domains.
If hosting is on Homepages (optionally) you can define a /htdocs subdirectory for each domain using the Domains Names control panel. For ccgi you would have to use .htaccess directives to achieve that.

[#3]

Thanks.  As a temporary measure could I do it until the domain becomes available?  In other words, with my plusnet user name, although I can't quite see how that would work.  Maybe you can only have 1 website based on your plusnet user name?

[#4]

The way hosting works any additional domains overlay onto the space allocated for your account/username. In the control panel if you define a particular domain (say mydomain.co.uk) to be in /htdocs/mydomain then www.mydomain.co.uk and www.username.plus.com/mydomain/ both point to exactly the same place in the file system.
So yes, you can prepare for adding a domain by using your account/username space with a subfolder.
When designing web pages it's up to you to make sure link references in each page are appropriate for the domain when serving that page. Avoiding fully qualified link references normally meets this requirement.